The present invention generally relates to the fabrication of electronic apparatuses and more particularly to the process for forming an acrylic resist on a surface of a copper layer that forms a part of a substrate such as a printed circuit board carrying thereon a conductor pattern.
Electroplating process is used commonly in the fabrication of multilayer circuit substrate of electronic apparatuses. In the fabrication of such a multilayer circuit substrate or a circuit substrate in general, an acrylic resist layer is applied on the surface of a copper layer that forms a part of the multilayer circuit substrate, wherein such an acrylic resist layer is patterned by a photolithographic process to form a resist pattern. The resist pattern thus obtained is then used as a mask pattern when forming an interconnection pattern on the substrate by an electroplating process.
FIGS. 1A-1G show the process of formation of a conductor interconnection pattern on a substrate.
Referring to FIG. 1A, a copper layer 2 is formed on an insulating substrate 1 by a sputtering process and the like, as an electrode. In the electroplating process, it should be noted that the surface of the substrate has to be covered by a conducting film for providing the same an electric conductivity. In other words, the copper layer 2 acts as an electrode. As most of part of the copper layer 2 thus formed has to be removed later, the copper layer 2 is generally formed as thin as possible.
Next, an acrylic resist layer 3 is formed on the surface of the copper layer 2 in the step of FIG. 1B, wherein the acrylic resist, being a mixture of a resin and a photosensitive agent or a photosensitive resin dissolved in a solvent, adheres upon the copper layer 2.
It should be noted that such a photoresist may be any one of: (1) a negative resist in which exposed part of the resist causes a polymerization to become insoluble to a developer solution while the rest of the resist remains soluble to such a developer solution; and (2) a positive resist in which exposed part of the resist alone causes a decomposition to become soluble to a developer solution. Generally, the negative resist is suitable for formation of fine patterns on a thin film multilayer substrate. On the other hand, such a negative resist generally has a poor adherence upon the underlying electrode layer 2. A typical example of the negative resist is the acrylic resist described above.
After the formation of the acrylic resist layer 3, the substrate 1 is subjected to a pre-exposure bake process, followed by an exposure process in the step of FIG. 1C such that the resist layer 3 is exposed by ultraviolet radiation. As a result of the exposure, the pattern of a photomask 4 is transferred upon the acrylic resist 3.
Next, a development of the resist layer 3 is conducted in the step of FIG. 1D, wherein the unexposed part of the resist layer 3 is dissolved by a developer solution.
After the foregoing step of developing, the copper layer 2 is connected to a negative pole of a d.c. power supply and an electroplating of copper is conducted. Thereby, the deposition of copper occurs only on the exposed part of the copper layer 2 and a thick copper pattern 5 is formed thereon. On the other hand no deposition of copper occurs on the resist pattern 3. Typically, the copper pattern 5 has a thickness of about 10 times as large as the thickness of the copper layer 2 used for the electrode.
After the electroplating process of FIG. 1E, the acrylic resist pattern 3 is removed in the step of FIG. 1F. In the state of FIG. 1F, it should be noted that the copper layer 2 remains exposed between the copper patterns 5, and the copper patterns 5 are interconnected by the exposed copper layer 2 electrically.
Thus, in the step of FIG. 1G, an etching process is conducted uniformly over the structure of FIG. 1F to remove the exposed copper layer 2, wherein the etching is conducted for a limited duration such that the etching is terminated upon removal of the exposed copper layer 2.
Thus, the acrylic resist is suitable for the formation of fine, minute interconnection patterns of copper or other conductive material on a thin film multilayer substrate. On the other hand, such an acrylic resist has a problem in that the adherence upon the underlying copper layer is generally poor as already noted. Conventionally, improvement of adherence of a resist upon an underlying layer has been achieved by adherence agent. For example, 1,1,1,3,3,3-hexamethyldisilazane has been used for such adherence agent of rubber-base resist or Novorak resist, while this substance is not effective for improving the adherence of acrylic resist. In the case of acrylic resist, no effective adherence agent is known so far.
In the absence of the effective adherence agent, conventional fabrication process of substrates has suffered from the problem in that the acrylic resist layer 3 tends to come off from the surface of the electrode layer 2 upon the post-exposure bake process that is conducted after the exposure step of the acrylic resist shown in FIG. 1D. When an electroplating process is conducted in such a state, it will be noted that the electrolytic solution containing copper penetrates into the gap between the resist layer 3 and the electrode layer 2 and causes a deposition of a copper layer therein as indicated in FIG. 2. Once a deposition occurs in such a gap with a thickness larger than the thickness of the electrode layer 2, the copper layer in the gap cannot be removed completely even after the etching process of FIG. 1G is conducted to remove the exposed layer 2. Thereby, a short-circuit occurs between the copper patterns 5.